1. Field of the Art
This invention relates to a heating furnace for use in the fabrication of electronic circuit modules each having a plural number of circuit devices mounted on a substrate, and to a method for fabricating electronic circuit modules by the use of such a heating furnace.
2. Description of the Prior Art
To meet the space- and energy-saving requirements in electronic appliances such as communication equipment, many efforts have been directed to reductions in size of various components of electronic appliances, including printed wiring board units, electronic circuit modules for the printed wiring board units and other electronic parts. Along with the reductions in size, the broad adoption of the so-called "chip on board" technology, i.e., the technology of directly mounting bare semiconductor chips on a substrate or baseboard, has greatly changed the method of fabrication of printed wiring board units. In this regard, there have been demands for the development of a method for fabrication of electronic circuit modules, which is capable of efficiently assembling electronic circuit modules employing the "chip on board" technology without impairing the reliability of semiconductor devices.
According to a typical conventional method, electronic circuit modules are produced in the manner as follows.
(a) After attaching a semiconductor device to a substrate by die bonding using silver paste, terminals of the semiconductor devices are connected to bonding pads formed on the substrate by wire bonding using, for example, gold wires. PA1 (b) In the next place, a synthetic resin material is applied to cover the semiconductor devices mounted on the substrate. PA1 (c) The substrate is put in a furnace to heat and cure the resin material. PA1 (d) Cream-like solder is applied, by silk screen printing or other suitable means, to the rear side of the substrate away from the front side having the semiconductor device. PA1 (e) Surface mounting devices are then put on the cream solder. PA1 (f) The substrate is placed in a furnace different from the one which was used in Step (c), for melting the solder and soldering the surface mounting devices in position.
In this manner, in the process of fabricating electronic circuit modules, the conventional method requires two independent furnaces, one being set at the curing temperature of the sealing synthetic resin material which covers the semiconductor device while the other one being set at the soldering temperature for the surface mounting devices. Namely, after curing the resin in one furnace, the surface mounting devices on the rear side of the substrate are set in position by reflow soldering in the other furnace.
Thus, the above-described conventional method involves complicate steps in the fabrication process of electronic circuit modules, and has a problem that, due to the time interval between the respective heating steps, the sealing resin material which covers the semiconductor device tends to absorb moisture before soldering the surface mounting devices on the other side of the substrate, causing rupture of the semiconductor device in some cases at the time of the reflow soldering. Besides, the moisture absorption by the resin material might result in its defoliation from the substrate. Even slight defoliation, which initially causes no trouble to the semiconductor device, might gradually deteriorate the semiconductor device over an extended period of time and give rise to a serious problem which is detrimental to the reliability of the electronic circuit modules.